Solid state imagers such as CCD, CMOS, and others, are in widespread use. FIG. 1 shows one exemplary CMOS imaging device 1 that includes a CMOS active pixel sensor (“APS”) pixel array 4 and a controller 6 that provides timing and control signals to enable the readout of image signals captured and stored in the pixels in a manner commonly known to those skilled in the art. Example arrays have dimensions of M×N pixels, with the size of the array 4 depending on a particular application. The imager pixels are read out a row at a time using a column parallel readout architecture. The controller 6 selects a particular row of pixels in the array 4 by controlling the operation of row addressing circuit 2 and row drivers 3. Signals stored in the selected row of pixels are provided on column lines to a readout circuit 7. The pixel signals read from each of the columns are then read out sequentially using a column addressing circuit 8. FIG. 1 shows that the readout analog pixel image signals are converted to digital values by an analog to digital converter 9 and then processed by an image processor 10.
One of the problems with digitally capturing an image is that portions of or an entirety of the captured image may be dimly lit either because of lighting conditions or a low exposure of imager pixels. Dimly lit image portions captured by a pixel array have a low signal-to-noise (SNR) ratio. One of the goals of image processing is to reduce noise in a captured image, particularly in the pixel signals from dimly lit or underexposed pixels.
Color correction is one of the processes applied by image processor 10 to captured image signals. Color correction typically involves adjusting the raw data from the sensor for colors to be correctly displayed on a standard output device such as a computer monitor. The same color correction technique, typically in the form of a color correction matrix, is applied to all pixel values of an image, including pixels which have low signal to noise ratios because of relatively low pixel values. The application of color correction typically deteriorates signal-to-noise ratio. Therefore, an improved color correction process which lowers noise in an image, in particular for pixels with low values and low signal-to-noise ratios, is desirable.